1. Field of the Invention
The present invention relates generally to the production of heat-shrinkable thermoplastic material and to the fabrication of films, sheets, and tubing of such material, as well as roll covers used on machinery rollers, such as paper machine rolls, textile rolls, food processing rolls, and lamination equipment.
2. Prior Art
Heat-shrinkable thermoplastics are used in many applications, among which are covering for machinery rollers in mill installations, such as in paper mills, where the rolls are used for guiding, spreading, and carrying the material being processed. These rolls are commonly provided with a polymer cover on their surface to resist corrosion and minimize friction and sticking. Various processes and materials for making and applying such tubes and roll covers are disclosed in, U.S. Pat. Nos. 3,050,786 to A. N. St. John et al, U.S. Pat. No. 3,225,129 to J. S. Taylor et al, U.S. Pat. No. 3,426,119 to F. M. Chapman et al, U.S. Pat. No. 3,481,805 to R. L. Holmes et al, U.S. Pat. No. 3,749,621 to J. P. Shoffner, U.S. Pat. No. 4,325,998 to H. S. Chapman, and U.S. Pat. No. 5,142,759 to J. Bonander et al.
A typical polymer cover of this type is in the form of Heat-Shrinkable Tubing (HST) commonly made from fluorinated ethylenepropylene copolymer (FEP), and less often from perfluoroalkoxy (PFA), a copolymer of tetrafluoroethylene (TFE) and perfluoropropyl vinyl ether (PPVE). The HST is presently fabricated by taking a suitable tube, welded from FEP sheet or fabricated by some other method, and applying pressure and heat to expand it hot, followed by cooling it in its expanded state to freeze the stress put into it by the expanding. The cooled tube is then shrunk onto the surface of the roll, again using the application of heat, to tightly shrink the tube or sleeve about the surface. The heating temperatures used with FEP HST are typically in the range of 170xc2x0 F. to 300xc2x0 F., well below its melt temperature of about 500xc2x0 F.
The melt processable FEP and PFA that are typically used in the HST application have been chosen over polytetraflurorethylene (PTFE), or other ultra high melt viscosity (UHMV) fluoropolymers, such as chemically modifiedPTFE, available as xe2x80x9cHostaflonxe2x80x9d from Hoechst AG of Burgkirchen, Germany, and commonly referred to as TFM, even though the latter materials have lower cost, greater strength, hardness, flex life, and other preferred physical properties and higher temperature use and service limits. This choice has been primarily due to the understanding and expectation in the art that the latter materials require higher temperatures to shrink. UHMV polymers are polymers that have a melt viscosity, which is too high for conventional thermoplastic processing, being of the order of a million times higher than conventional polymers that are suitable for melt processing. For example, PTFE has a melt viscosity of 101 l poises while FEP has a melt viscosity of 104 to 105 poises. Another UHMV polymer that has not been used in the HST application is ultra high molecular weight polyethylene or UHMWPE, which is an extremely high density polyethylene with a molecular weight range of 3,000,000 to 6,000,000. This compares with a molecular weight range of 300,000 to 500,000 for high molecular weight polyethylene (HMWPE), which can be readily melt processed. Again, the melt viscosity of UHMWPE is too high for conventional melt processing. Thus, while PTFE and other UHMV polymers have many superior qualities and, in addition, may be much less costly, still heretofore, FEP has been regarded as the preferred material in implementing this shrink technique.
Expanded UHMV polymer tubes such as those made from PTFE are known,but they have not been satisfactorily employed in the HST shrink process since state of the art HST made from PTFE is typically heated to near or above the gel temperature of PTFE, 621xc2x0 F., first for expansion and again, after cooling, to effect complete recovery, i.e., shrinkage of the tube onto the roll. These temperatures are so high as to pose a danger of causing thermal damage to the substrate roll and as a practical matter they cannot be easily accomplished on larger samples with the application of energy from a simple tool, such as a hot air gun currently used with FEP HST. Moreover, during heating of the HST to the gel temperature for recovery there is a tendency for one section of the tube to be overheated while the remainder of the tube is too cool to shrink on the roll. This leads to the problem of non-uniform recovery/shrinkage of the tube. Although in the above-noted U.S. Pat. No. 3,050,786 it is taught that PTFE recovery can be accomplished at lower temperatures, as low as 300xc2x0 F., this is at the expense of process time, particularly when rapid cooling is used.
Problem to be Solved:
Developing a system and method for producing heat-shrinkable film, sheets, and tubing with optimal properties and ease of processing and installation for facilitating use such as in making HST and in other suitable applications.
Objects:
It is accordingly an object of the present invention to utilize the discovery that a conventional material, having heretofore unappreciated capabilities, can be fabricated with a comparatively simple process to achieve improved heat-shrinkable film, tubing, and roll covers.
It is another object of the present invention to reveal the superiority of PTFE, and other UHMV polymeric materials such as TFM, and UHMW polyethylene, over FEP and irradiated HMWPE in the heat-shrinkable tubing (HST) process for covering machine rolls and other components.
It is a further object of the invention to provide improved heat-shrinkable tubes of TFE, TFM, and UHMWPE materials, reinforced with conductive and other additives, as covers for rolls, bars, tubes, pipes, and other elements of a relatively constant circumference, as well as improved heat-shrinkable sheets and film of these materials, particularly heretofore unrealized UHMWPE heat-shrinkable film.
The present invention involves the discovery of the suitability and advantages to be achieved with the use of PTFE and other ultra high melt viscosity (UHMV) polymers such as TFM, and UHMWPE, as heat shrinkable material, and their use, rather than FEP or PFA, in the HST process. It has been found, for instance, that PTFE, typically fully sintered PTFE used in HST applications, may be readily substituted for FEP in the typical HST process with little significant change in the existing process steps. Appropriate expansion of a PTFE tube may be obtained with pressure and heat treatment at temperatures less than 300xc2x0 F., preferably in the range from about 190xc2x0 F. to about 250xc2x0 F., in a matter of a few minutes, and the resulting tube is sufficiently stable to be shipped for later shrink application at comparable temperatures onto a roll at a mill. Also, UHMV polymers may be processed in the form of a sheet or a film to produce improved heat-shrinkable material that is shrinkable at comparatively low temperatures in a minimum time, particularly heretofore unrealized UHMWPE heat-shrinkable film. The tube and the sheet or film are shrinkable in at least one dimension and may be shrinkable in two dimensions.
A typical HST process begins with the provision of a tube which may involve, first, cutting a sheet of ultra high melt viscosity (UHMV) polymer film, e.g., of PTFE, TFM, or UHMWPE, to the appropriate circumference and length to fit the roll to be covered, as is done with FEP. The edges of the cut sheet are then joined, such as by fusion welding, to form a tube with a seam that is strong enough to be expanded. This tube is then expanded to a sufficient size to fit about the surface of the roll, by closing its ends and placing it in an expansion housing or sizing chamber, such as a cylindrical pipe that will accept the UHMV tube and determine the diameter to which it will expand. The tube is expanded to that diameter using pressure and, if desired, heat, such as steam heat at a temperature of about 200xc2x0 F., or, depending on the material thickness, to a temperature somewhere above the expansion temperature but below the gel temperature. The expanded tube is cooled to ambient temperature at the expanded diameter and is sufficiently stable upon cooling to be stored for later shipment and use. After shipment, the tube is slipped over the roll surface to be covered and, when positioned properly, the tube material is heated, e.g., using hot air guns, a gas torch, a heat blanket, or an oven, to a temperature sufficient to shrink it into tight contact with the surface, e.g., a temperature somewhat higher than the temperature at which it was expanded, typically at about 225xc2x0 F. The shrink time is typically less than 3 minutes. The size of the circumference of the tube is chosen initially to be about 0.50 to about 0.95 times, and preferably from about 0.80 to about 0.92 times, the roll size. The resulting roll cover has improved mechanical properties as well as being amenable to being reinforced with suitable particulate additives such as a conductive additive to provide anti-static properties and additives to improve its wear, load carrying, and compressive strength, and to reduce its thermal expansion, as are variously known in the art.